The development of ballast's for operating ionic conduction lamps has progressed from conventional ballasts that operate at a low 60 Hertz frequency to those that operate at frequencies from 10 Kilohertz to 40 Kilohertz.
The low frequency ballast is generally a series reactor transformer which includes a large number of windings. Thus, the ballast acts as a inductive device that serves to both ignite the lamp and to also limit the current to the lamp. Immediately after the lamp is ignited, the impedance of the lamp drops to a very low level and, hence, it is necessary to limit the current after ignition in order to avoid burning the lamp. The inductive reactance in the ballast operates to limit the current after ignition of the lamp.
There are many disadvantages inherent in the conventional low-frequency ballast. One is the weight and size factor of the ballast. Due to the heavy transformer, provisions must be made in each lamp fixture to mount and support the weight of the ballast. Another, because of the coil and core design of the transformer, the ballast will not start at temperatures below 52.degree. F. (11.1.degree. C.).
The transformer core in the ballast often tends to vibrate and generate a hum in the audible frequency spectrum. While this hum may not have a great amplitude, it is, nevertheless, distracting and uncomfortable. In combination with the hum, the ballast also produces a low frequency "strobe effect" that causes irritation and headaches in many persons. The strobe effect is particularly noticeable at the peripheral vision of the eyes.
Large capacitors are often times required to correct the power factor and phase displacement. These capacitors are relatively expensive due to their size and thus substantially increase the overall weight and cost of the ballast.
The inductive device in the ballast often generates a significant amount of heat. In many cases, when the lamp is not mounted in an environment where air flow can dissipate the heat, other means must be employed to dissipate this heat. Additionally, if the ballast is operated for an excessive period of time, the heat buildup may damage the ballast necessitating replacement.
One of the significant disadvantages of conventional ballasts is that the ballast requires a substantial amount of electrical power for its operation in order to ignite and thereafter maintain energization of the lamp. A substantial amount of power is required to ignite the ionic conduction lamp and after the lamp has been ignited, a lesser but continuing current source is applied to the lamp in order to maintain its energization. The high-frequency ballast is discussed in the following U.S. patents which do not read on the claims of the instant invention but are indicative of the present state-of-the-art:
______________________________________ U.S. Pat. No. INVENTOR ISSUED ______________________________________ 4,286,194 Sherman 25 August 1981 4,005,335 Pepper 25 January 1977 3,889,153 Pierce 10 June 1975 3,396,307 Campbell 6 August 1968 ______________________________________
The Sherman patent discloses a ballast that functions at an operating frequency within the range of 22 to 25 Kilohertz. The ballast is designed to start and maintain energization and operation of a load which has a relatively high impedance during starting and a substantially lower impedance after starting and during operation. The load is generally an ionic conduction lamp including a phosphor excitable lamp such as a fluorescent lamp. The inventor of the instant improved ballast has the rights to the Sherman patent. In the process of experimenting with the design several problems were uncovered which led to the improved ballast described infra. Three of the most notable problems are described below:
The transistor 42 and/or the diode rectifier bridge 26 were subject to catastrophic failure due to excessive voltage spikes generated within the circuit. PA0 To operate the ballast it is necessary that current be applied through the heating filaments 52 of the fluorescent lamp L1 and L2. Thus, if a filament opens the ballast circuit is inoperative. PA0 Because of the lamps high impedance at the start of the lamps ignition and the lower impedance after starting two impedance matching circuits are required. PA0 operates over a wide range of input power requirements including a d-c power input, PA0 operates at a high frequency that avoids the "strobe effect" that is prevalent in low-frequency devices. The strobe effect can cause irritation and headaches in many persons, PA0 can be designed, with the proper selection of components, to light one or a plurality of ionic conduction lamps, PA0 is ten times lighter than conventional low frequency ballasts. Conventional ballasts weigh 5 pounds (2.3 Kg) whereas the instant ballast weights 8 ounces (0.23 Kg). PA0 can include a variable resistor that allows the lamp to be dimmed and operated at a lower power than is presently required for current ballasts.
In addition to the above problems, the Sherman patent has no provision to dim the lamp. This feature is particularly useful when less illumination is preferred and also provides a cost saving since less power is consumed with a lower light level.
The Pepper patent discloses a high frequency power source for fluorescent lamps. The device includes an inverter and an oscillator circuit that includes a transistor and a transformer that is connected to the lamp. A detector circuit is connected across the transistor output developed at one of the windings of the transformer. The circuit develops a control signal that varies as a function of the transistor output. The control signal is connected to the base of the transistor which is in parallel with the output of a feedback winding. When the control signal exceeds a predetermined value, the transistor output is changed to correspond to the required load.
The Pierce patent also discloses a high-frequency high-voltage power source for fluorescent lamps. The device includes an inverter with an oscillator and a transformer. The oscillator circuit comprises a transistor-set that has its emitter and collector electrodes connected in series with the primary winding of the transformer. The base of the transistor is connected across the transformer feedback winding. The circuits provide a lamp starting voltage and a reduced voltage after starting and during operation.
The Campbell patent discloses a fluorescent lamp ballast comprising a single transistor inverter circuit that allows the lamp to be operated from a direct current source. The transistor is connected, in series with the primary winding of an auto-transformer, across the d-c supply with a feedback connection from the transformer secondary to the transistor base electrode. The lamp load is connected in series with a capacitor across the transformer's secondary winding. A shunt capacitor is connected across the transformer secondary for load regulation under open circuit conditions when excessively high voltages might damage the transistor.